Valley curves at night – headlight sight distance criterion For safe night driving, the length of a valley (sag) curve should be designed so that the headlight beam distance equals which standard sight distance?

Introduction / Context:
Valley (sag) curve design must ensure that at night the light from a vehicle’s headlamp adequately illuminates the roadway ahead. The primary safety requirement is that a driver can see far enough to stop safely if an obstacle appears.

Given Data / Assumptions:

• Standard headlight mounting height and upward divergence are assumed per design guidelines.
• Design speed dictates required sight distance.
• Night condition governs control for sag curves due to limited headlight illumination over a concave vertical profile.

Concept / Approach:
For sag curves at night, the available sight distance is limited by the distance the headlight beam can reach the pavement surface. Therefore, the length of the curve is chosen so that the headlight distance is at least equal to the stopping sight distance (SSD) corresponding to the design speed, ensuring the driver can stop within the illuminated roadway.

Step-by-Step Solution:

Verification / Alternative check:
For crest curves, SSD is limited by line of sight over the curve; for sag curves at night, SSD is limited by headlight reach. In both cases, SSD, not overtaking sight distance, is the control for ordinary design.

Why Other Options Are Wrong:
Overtaking sight distance is not the design control for sag curves at night; it is a separate daytime maneuvering requirement. Combining or differencing SSD and OSD has no basis in standard practice.

Common Pitfalls:
Using overtaking sight distance as a universal control; ignoring headlight mounting height and beam angle assumptions.

Final Answer:
Stopping sight distance